#include <bits/stdc++.h>
#define ll long long
using namespace std;
ifstream fin("ordine.in");
ofstream fout("ordine.out");
string s, rez;
int fv[130];

int main() {
    fin >> s;
    for (int i = 0; i < s.size(); ++i)
        ++fv[s[i]];
    for (int i = 'a'; i <= 'z'; ++i) {
        int j = i + 1;
        while (fv[i] > 0) {
            while (fv[j] == 0)
                ++j;
            bool adauga = true;
            if (rez[rez.size() - 1] != i)
                rez += i, adauga = false;
            if (j <= 'z')
                rez += j;
            if (adauga)
                rez += i;
            --fv[i], --fv[j];
        }
    }
    fout << rez;
    return 0;
}